#### Transcript Work and Energy Notes

Bell Ringer What do you think of when you hear the word energy? (List at least three items in your Bell Ringer) Bell Ringer 10/25 What is another term for the ability to do work? Energy Energy: The ability of an object to do work Units: Joules (J) Types of energy include: Mechanical: Energy of movement and position Chemical: Energy stored in chemical bonds of molecules Energy Thermal: “Heat energy” stored in materials at a certain temperature Nuclear: Energy produced from the splitting of atoms Radiant Energy: Energy traveling the form of electromagnetic waves Electric Energy: Energy traveling as the flow of charged particles (i.e. electrons) Work Work is done when a task produces a change in energy Factors affecting work done: The application of a force The movement of the object by that force over a distance Bell Ringer How much work is required to lift a 2kg object 2m high? Work Therefore: Work = Force x Distance W = Fd Units: Joule (J) 1 J = 1 N.m Note that work requires a distance Bell Ringer 3/31 What is another term for the ability to do work? You push a stationary wall with a force of 1000N. How much work was done to the wall? Bell Ringer Power How much work is performed over a period of time Therefore: Power = Work / Time P = W/t Units: Watts (W) where 1 W = 1 J/s Thought Question How many horses are in one horsepower? Power Power can also be converted to units of horsepower (hp) Note: 1 hp 750 W coffee maker blender lawn mower Corvette 0.75 hp 1.5 hp 5-6 hp 400 hp Bell Ringer If Superman, at 90kg, jumps a 40m building in a single bound, how much does Superman perform? If this occurs in 3s, what is his power output? Energy The amount of work done by an object does not depend on the path taken Work depends only on the object’s starting and ending points As work is done on an object, the object itself gains the opportunity to do work Energy For example: A bowstring drawn back on a bow Winding an alarm clock Raising the arm on a pile driver All of these objects now have the ability to do work Mechanical Energy Mechanical Energy: Energy of movement and position There are two major types of mechanical energy: Potential Energy: Energy of position Kinetic Energy: Energy of motion Potential Energy Gravitational Potential Energy: The potential due to elevated positions P.E. = mass x gravity x height P.E. = mgh Recall: weight = mass x gravity Therefore: P.E. = weight x height Potential Energy Kinetic Energy Objects in motion are capable of doing work . . 2 KE = ½ mass velocity KE = 2 ½mv Kinetic Energy Note that the velocity of the object is squared when determining KE If the velocity of the object is doubled, the KE is quadrupled Energy Conservation Energy is constantly transforming, but never “disappears” Law of Conservation of Energy: Energy cannot be created or destroyed, only changed from one form to another. Energy Conservation Potential and kinetic energy are constantly transforming back and forth Most of the time during this transformation, some energy is turned to heat and transferred out of the system Energy Conservation Bell Ringer Jill has a velocity of 5m/s. If she has a mass of 60kg, what is her kinetic energy? If Bob, at 70kg, is standing on top of a 13m high hill. What is his potential energy? Work-Energy Theorem The change in gravitational potential energy of an object is equal to the amount of work needed to change its height Therefore: Work = DPE Fd = mgh Work-Energy Theorem The KE of a moving object is equal to the work the object is capable of doing while being brought to rest Therefore: W = DKE or Fd = 2 ½mv Work-Energy Theorem Putting these two ideas together gives us the general Work-Energy Theorem: If no change in energy occurs, then no work is done. Therefore, whenever work is done, there is a change in energy. Bell Ringer List and give an example of the 6 types of simple machines. Simple Machines Machine: A device used to multiply forces or to change the directions of forces There are six types of simple machines: Pulley: Grooved wheels which assist in raising, lowering, or moving an object Simple Machines Lever: A stiff bar which pivots on a support to assist in lifting or moving an object Wedge: An object consisting of a slanting side ending in a sharp edge which separates or cuts materials apart Wheel and Axle: A wheel with a rod through its center which lifts or moves objects Simple Machines Inclined Plane: A slanting surface connecting a lower level to a higher level Screw: An inclined plane wrapped around a rod which holds objects together or lifts materials Bell Ringer What is an example of a 100% efficient machine? Mechanical Advantage Mechanical Advantage: A machine’s ratio of output force to input force Mechanical Advantage = Output Force Input Force i.e. A machine which outputs 80 N for every 10 N you put in has a mechanical advantage of 8. Note that the load will move only 1/8 of the input distance Efficiency Efficiency: A machine’s ratio of useful work output to total work input Efficiency = Useful Work Output Total Work Input Efficiency is expressed as a percent i.e.) An efficiency result of 0.25 means 25% efficiency Efficiency Ideal machines have 100% efficiency This means that all of the energy put into the machine exits as useful energy All other machines will ALWAYS have an efficiency of less than 100% A machine cannot output more work than is put into it Pulleys Single Pulley: Changes the direction of a force exerted by a rope or cable System of pulleys: Multiplies input forces, creating large output forces Pulleys • Each supporting strand of rope holds an equal fraction of the weight • Tension in this cable is the same throughout its entire length • Input force = tension in each supporting segment of the cable • Mechanical advantage = number of supporting strands Pulleys Input force = 30 N 30 N Pulleys Input force = 15 N 30 N Bell Ringer How many supporting strands are there ? What is the Mechanical advantage here equal to? What is the input force required to lift the 200kg object? More Practice What is the minimum effort that must be applied to lift the load? For every 2 meters the rope is pulled through what height does the load rise off the ground? What is the mechanical advantage? LEVERS Levers A simple machine made of a bar which turns about a fixed point Fulcrum: The pivot point of a lever Change the direction of or multiply input forces Three Types of Levers Type 1 Lever: Fulcrum lies between the input force and the load i.e.) A seesaw Type 2 Lever: The load lies between the fulcrum and the input force i.e.) A pry bar Three Types of Levers Type 3 Lever: The input force lies between the fulcrum and the load i.e.) Your forearm pivoting about your elbow Lever Lab Levers If friction is small enough to neglect: Work Input = Work Output or (Fd)input = (Fd)output Therefore: A small input force over a large distance will output a large force over a small distance Levers Levers Wedge Wedge: An object consisting of a slanting side ending in a sharp edge which separates or cuts materials apart i.e. knife Wheel and Axel Wheel and Axle: A wheel with a rod through its center which lifts or moves objects ie: Cart Inclined Plane Inclined Plane: A slanting surface connecting a lower level to a higher level i.e. Accessibility ramp Screw Screw: An inclined plane wrapped around a rod which holds objects together or lifts materials Compound Machine Compound machines use two or simple machines to complete a task Examples? Rube Goldberg Device Bell Ringer How much energy is transferred in lifting a 5 kg mass 3m? What is the work energy theorem?